Update docs about an existence of .elementType (#2145)

Hi, I've met the same issue on [here](facebook/prop-types#200) at storybook and found the following [PR](facebook/prop-types#211) that adds the `elementType` feature. It could find the doc on npm, but not the official react site.

content/docs/typechecking-with-proptypes.md

@@ -57,6 +57,9 @@ MyComponent.propTypes = {
   // A React element.
   optionalElement: PropTypes.element,
 
+  // A React element type (ie. MyComponent).
+  optionalElementType: PropTypes.elementType,
+
   // You can also declare that a prop is an instance of a class. This uses
   // JS's instanceof operator.
   optionalMessage: PropTypes.instanceOf(Message),